Adds a Help > UI Introduction, which highlights basic aspects of the Qt
Creator UI.
The pages to show are defined in a format that can later move into a
text based configuration file like JSON. It specifies an optional object
name of a widget to highlight, a title, a brief description and a longer
description (potentially with tables, lists and other simple HTML).
Fixes: QTCREATORBUG-21585
Change-Id: Idb64c87e1d752bc24437588278093a96be0eeddb
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
